To control a brushless DC motor, it is necessary to control a motor drive signal according to a current position of the motor by reading positional information through a hole device. In this case, a differential signal formed of sign waves called a hole signal is received by a hysteresis comparator.
However, the hole signal is easily affected by common mode noise. Accordingly, it is desired to receive the signal by a high performance hysteresis comparator which has a fine hysteresis characteristic of around 30 mv. However, it is difficult to achieve such a high performance hysteresis comparator having the fine hysteresis characteristic of around 30 my under poor conditions, in which the operation of the comparator is affected by, for example, fluctuations in power supply voltage and imperfections in the manufacturing process.
Conventionally, a hysteresis comparator has been proposed which adjusts a hysteresis characteristic with high precision over a wide range. The conventional hysteresis comparator includes a differential pair, switches to change the voltages around the differential pair based on the output signal of the comparator, and resistors provided by connecting in series to or in parallel with the differential pair to control a hysteresis amount based on the output signal of the comparator.
In the conventional comparator described above, although voltage variations caused by fluctuation of the resistance of the differential pair may affect the hysteresis characteristic significantly, no consideration is given to eliminating the effect of such fluctuation of the resistance. Further, the resistance of the switches themselves also affects the hysteresis characteristic significantly. Accordingly, it is difficult to achieve a desired characteristic of the comparator.
Further, since the output signal is used as a control signal to change the connection of the switches, the comparator may generate an erroneous output signal when the feed-back signal is delayed significantly due to a wiring delay during feedback of the output signal.